The nature of gas turbines is such that they emit small amounts of undesirable pollutants into the surrounding atmosphere. Although smoke, excess carbon monoxide and unburned hydrocarbons all constitute undesirable pollutants in the exhaust of state-of-the-art gas turbines, it is the emission of excess amounts of nitrogen oxides (NOx) and carbon monoxides (CO) which causes particular concern, owing to the adverse affects attributed to these gases. Thus, it becomes particularly desireable to provide a gas turbine combustion system which operates with a minium amount of undesirable exhaust emission.
It is well known that lowering the temperature of combustion will decrease the concentration of nitrogen oxides in the turbine exhaust gases. It has also been demonstrated that buring the turbine fuel with excess air i.e., using a fuel-lean mixture in the combustion process, will accomplish such a temperature reduction. However, the leaness of the fuel-air mixture required to effect a flame temperature reduction at full turbine load may not support a satisfactory flame under low load or start-up conditions. When the latter conditions prevail, the turbine may operate at poor combustion efficiency, or not at all, if the fuel-air mixture used is the same fuel-air mixture used at full load. Incomplete burning of the fuel mixture will occur, resulting in the presence of excessive amounts of carbon monoxide and unburned hydrocarbons in the turbine exhaust.
In U.S. Pat. No. 4,297,842, assigned to the assignee of the present invention, it has been recognized that Nox control can be implemented by dividing the flow of compressor air to the combustor in two separate flows to the reaction zone and the dilution zone respectively and adding a NOx suppressant to the reaction zone airflow. However, it is noteworthy that the respective airflows between the reaction zone and the dilution zone is fixed by the combustor design.
In U.S. Pat. No. 4,255,927, assigned to the assignee of the present invention, provision is made for changing the flow volume of compressor air between the reaction zone and the dilution zone of a combustor. There are significant hardware requirements including ducting and a mechanical valve for distributing the flow of air between the two zones.
Another significant factor in the control of NOx is the time of the reaction referred to as "residence time". A shorter residence time will produce less NOx by limiting the exposure of free nitrogen and excess oxygen to combustion temperatures but a longer residence time may be needed to assure complete combustion to avoid the production of excessive hydrocarbons. It is desireable to be able to adjust residence time to an optimum for the pupose of minimizing the production of NOx without adversely affecting flame stability under changing loads and fuel flows.
It is therefore and object of the present invention to provide an improved combustor linear for minimizing the formation of NOx.
It is another object of the invention to provide a combustor liner effective for facilitating the control of NOx having a minimum of hardware changes from a conventional combustor liner.
It is another object of the invention to provide a combustor liner effective for controlling the formation of NOx for a gas turbine combustor which is passive (requiring no separate control) and reliable.
It is yet another object of the present invention to provide a combustor liner which will increase the turndown ratio of the gas turbine, i.e., the operational range of the gas turbine under which satisfactory performance is achieved.
These and other objects of the present invention, together with the features and advantages thereof will become more apparent from the following detailed description when read in combination with the accompanying drawings.